(1) Field of the Invention
The present invention relates to the exercise of control over the quantity of at least a pair of constituents of a mixture so as to satisfy quantity and/or temperature requirements dictated by equipment located downstream of the mixing point of the constituents. More specifically, this invention is directed to an air flow and temperature control for a pulverizing mill of the type which receives undried coal and discharges a fuel stream consisting of dry pulverized coal entrained in air. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in the exercise of control over a pulverizing mill of the type employed by utilities to grind coal, delivered thereto from storage bunkers which may be exposed to the weather, to provide a fuel stream for injection into the furnace of a steam generator. Techniques and apparatus for controlling the flow rate and temperature of primary air which dries and then conveys fuel from a pulverizing mill to the burners of a vapor generator furnace are known in the art. The prior art controls are exemplified by U.S. Pat. No. 2,831,637 to H. C. Mittendorf et al and U.S. Pat. No. 3,273,520 to R. D. Hottenstine.
The air entering a coal mill or pulverizer must satisfy two basic criteria. First, it must be of sufficient quantity to provide adequate transport velocity for the fuel stream which exits the pulverizer. Secondly, the air must have sufficient heat content to dry the coal in the pulverizer. The air delivered to a pulverizer is known in the art as "primary air". In the prior art, as well as in the present invention, a portion of this primary air is at ambient temperature and is referred to as the "cold" air. The remainder of the primary air passes through an air heater and is termed the "hot" air. Upstream of the pulverizer, the hot air, which passes through a regulating damper, and the "cold" air, which also passes through a regulating damper, are mixed. The quantity and temperature requirements of the primary air stream are met by adjusting the dampers in the hot and "cold" air supply conduits.
In the prior art the temperature of the fuel stream leaving the pulverizer was measured and compared with a desired value. Simultaneously, the quantity of air entering the pulverizer was sensed and also compared with a desired value. The net effect of any temperature errors and air flow errors was applied to both the hot and "cold" air dampers which were repositioned until both temperature and air flow returned to their desired values. In exercising this control in the prior art, an air flow error would cause both dampers to move in the same direction whereas a temperature error would cause the hot air and "cold" air dampers to move in opposite directions. Thus, in summary, in the prior art each of the hot and "cold" air dampers has been operated in response to both air flow and temperature errors. As will be discussed in more detail below, this is not altogether desirable since the characteristics of the two variable; i.e., air flow and temperature; are different. For example, flow control may be effected at a much faster rate than temperature control. Accordingly, it has been common practice in the art to "tune" both controllers whereby their reaction times would be commensurate with the average rate of change of temperature which is the variable which changes at the slowest rate.
The prior art pulverizer air flow and temperature controls provide satisfactory performance when transients are slow and the primary air system is able to simultaneously satisfy both air flow and temperature requirements. However, should a conflict arise, the prior art systems are not capable of determining which variable; i.e., temperature or flow; should dominate. Transients may, for example, result from a sudden change in coal moisture incident to a change from one supply bunker to another. An operating transient may also occur if the load on the air heater changes as a function of the number of pulverizers in operation. Poor transient response may be manifested by a failure to maintain the minimum required air flow. Should the air flow momentarily fall below that level required to transport the coal, some of the pulverized coal will "fall out" in the pipes downstream of the pulverizer thus presenting a fire hazard when hot air later flows through the pipes. If the temperature is insufficient to dry the coal, the coal will not be ground properly and the pulverizer will "spill"; i.e., the coal will not be entrained in the air stream. Obviously, if the input air temperature to the pulverizer is too high, a fire hazard is presented. As implicit in the preceeding remarks, pulverizer operation requires that air flow be kept above a minimum commensurate with transport velocity while maintaining a temperature which is adequate to dry the coal but not so high as to present a fire hazard.
Prior pulverizer air flow and temperature controls have, in addition to poor transient response as discussed above, possessed operating limitations. Again by way of example, if insufficient hot air was available, the hot air damper was driven to the open setting as the control tried to maintain air flow and temperature at the chosen values. When the desired values of both air flow and temperatue could not be achieved or maintained, prior pulverizer controls would function so that either one or the other of the variables; i.e., either temperature or air flow; would dominate. The parameter which dominates, however, is not always the right parameter taking all of the operating conditions into account.